I'm a very long time linux user (I think first install was Mandrake around 1998) but haven't owned a linux PC in well over a decade (use MBPs for work, linux on the cloud and in docker). I also have an older iMac that my kids use, and I dual boot to Windows via bootcamp for the only game I play (Civ V).
With Civ 7 coming out next month the crew I play with want to give it a go, which makes me realize I'll need to upgrade my desktop (or buy a gaming laptop).
I've been looking at the System76 Thelio Mira line but they won't ship until end of Feb according to their website. Trying to find something prebuilt with Linux already on it (I don't care about distro/will likely change it anyway, but want to make sure everything is supported "out of the box"). Preferably under $2000 but I realize it might be difficult esp with an Nvidia card.
Other than playing Civ I'll probably use it for some OSS development and personal projects (including toy deep learning stuff, anything really needing much horsepower would also be in the cloud), as a media center for my house, etc.
Any suggestions for specific machines or vendors to check out? Anything to watch out for? (Because of crossplay I assume I'll have to run Steam in Proton)
I usually recommend pcpartpicker.com to pick out the parts themselves. I've used it to build a few computers, and it makes it very easy to decide what you want.
As for Linux compatibility, many things should work right out of the box. I would agree with the other person that mentioned the motherboard being the main thing to watch for. Other than that, Nvidia GPUs would be the only thing I would avoid, just because they don't have drivers directly in the Linux kernel yet. You can install them yourself or use a distro that has them already built in, but it's just one more thing you might need to work through.